1. Field of the Invention
The present invention generally relates to a heat exchanger and, more particularly, to a heat exchanger for use with high temperature materials.
2. Description of the Prior Art
In a typical propane or gas-fired hot air furnace, burner assemblies within the furnace inject a mixture of fuel and air into the inlets of a respective number of primary heat exchanger assemblies. After the fuel air mixture is combusted within the primary heat exchangers, the combustion gas travels through a serpentine flow path within the primary heat exchanger assemblies, exchanging some of the heat produced to the room air.
The more efficient gas-fired hot air furnaces increase the amount of heat energy transferred from the flue gas to the air to be heated. One manner in which the efficiency of the gas-fired hot air furnaces is being raised is by cooling the flue gases while still within the furnace to below their dew point. By cooling the flue gases to the point where condensation occurs, the latent heat of evaporation may be recovered as usable energy.
The recovery of the latent heat of evaporation is typically accomplished by adding a condensing heat exchanger to the primary heat exchanger and by passing air to be heated over the condensing heat exchanger and then through the primary heat exchanger. Some typical heat exchangers have been constructed from two engineering metal sheets such that a fluid flow is created when the two sheets are stamped and assembled.
As with the primary heat exchanger, the condensing heat exchanger must be constructed from a material having good heat transfer, adequate strength, minimum material thickness and preferably low manufacturing costs. The condensing heat exchanger, however, must additionally be constructed from a material having a high resistance to chemical attack. When the combustion gases condense within the condensing heat exchangers, a variety of acids may be produced, including carbonic and nitric acids, which can severely corrode bare steel and pit aluminum and copper with concentrations as little as 10 ppm (parts per million). As should be apparent, a condensing heat exchanger must be carefully designed for the environment within which the exchanger is placed.
Many condensing heat exchangers have been constructed from such materials as 300 Series stainless steel, which is a rather expensive material. Some less expensive engineering materials have been used with coatings that have been applied from a liquid or powder state. These coated engineering materials, however, have performed very poorly when used as a condensing heat exchanger since the coatings blister, crack, and fall off during the forming process of the condensing heat exchanger or while in service, thereby causing localized corrosion of the steel substrate.
An improved condensing heat exchanger is disclosed in U.S. Pat. No. 4,738,307 to Bentley, the disclosure of which is hereby incorporated by reference. The condensing heat exchanger in Bentley is formed from a single sheet of metal stamped to have an inlet, an outlet, and a flow passage between the inlet and outlet. The stamped sheet is laminated with a corrosion resistant material, preferably polypropylene, is folded at a center line, and then tabs on the sides of the exchanger are folded over and crimped to form the completed condensing heat exchanger. Because the single sheet is folded, one edge of the condensing heat exchanger is seamless, thereby reducing the risk of condensate leakage from the condensing heat exchanger.
While the condensing heat exchanger of the type disclosed in Bentley is less expensive than one constructed from Series 300 stainless steel, the condensing heat exchanger is still rather expensive. The use of polymer coated steel in Bentley's condensing heat exchanger is more expensive than many other types of materials, such as many plastics. The process for constructing Bentley's condensing heat exchanger is also lengthy since it involves a multi-step process including the steps of stamping, laminating, folding the two halves of the sheet, folding the tabs, and crimping the tabs.
With some heat exchangers in general, the heat exchangers have been constructed from polymers rather than stainless steel. For instance, U.S. Pat. No. 4,790,372 to Gemeinhardt et al. and U.S. Pat. No. 4,947,931 to Vitacco both disclose heat exchangers having passages formed from a thermoplastic or nylon polymer. If these heat exchangers were used as a condensing heat exchanger in a gas-fired hot air furnace, the heat exchangers would have to be constructed from a high temperature polymer material, which is rather expensive, in order to withstand the high inlet temperatures of the combustion gases. Thus, although a plastic material has a high corrosion resistance, a condensing heat exchanger constructed from a high temperature resistant polymer alone would not offer any cost savings.
A polymer heat exchanger, which would likely have a metallic header, would have other disadvantages as well. For instance, many furnaces have a variable speed room side blower. At low heating loads, special thermostats control the fan speed and burner firing rate so that they are at a reduced level, thereby increasing energy efficiency and comfort for the occupants by reducing the amount of noise through the ductwork. At these low heating loads, the condensation point of the combustion products is moved closer to the entrance of the secondary heat exchanger. This shift in location of the condensation point could expose the metallic header to the corrosive acids which are capable of rapidly degrading mild steel. The location of the condensation point can also shift in a non-variable speed condensing furnace, such as when the room air is below normal temperature. Thus, a need exists for a secondary heat exchangers which can accommodate location changes in the condensation point.
Another problem of a polymer exchanger having a metallic header relates to multi-poise operation. It is desirable in the home heating industry to produce furnaces which can be installed in a wide variety of orientations, such as horizontal right, horizontal left, vertical up, and vertical down, also known as multi-poise. A heat exchanger that can operate in the wide variety of orientations reduces the need to manufacture and stock furnaces designed for only one orientation. A secondary condensing heat exchanger, however, must accommodate for the flow of condensates through the heat exchanger and to a drain. Due to variations in orientation as well as other variations in the operation of a furnace, the condensates may likely flow down into the metallic header portion of the exchanger thereby degrading the header.
Other types of heat exchangers have been constructed from ceramic materials, such as glass. For instance, U.S. Pat. No. 4,653,575 to Courchesne describes an air-to-air heat exchanger comprised of a plurality of glass tubes through which the heated air travels. The ceramic materials, such as glass, are desirable since they can withstand generally higher temperatures than many plastic materials. For instance, U.S. Pat. No. 4,768,586 to Berneburg et al. discloses a ceramic heat exchanger which has a better heat resistance and corrosion resistance than most metallic exchangers. The ceramic materials, however, are more fragile and brittle and more easily crack or break during shipping, installation, or operation. Consequently, ceramic materials are not commonly used in many heat exchangers.
Therefore, it is still generally a problem in the art to provide a low cost heat exchanger which has a good heat transfer, adequate strength, and minimum overall system cost. It is further a problem in the art to provide a low cost condensing heat exchanger which can withstand both the high temperatures of the combustion gases and the corrosive acids within a gas-fired hot air condensing furnace.